POLYMORPHISM IN C .nermoralis B. Dixon-Naraine, C. Kuffour , J. Jones-Alleyne, J. Rogers, K.Oseni, V. Julien Queen Mary University of London INTRODUCTION
Polymorphism is the phenomena where two or more phenotypes are observed in the same species. It is a natural process that promotes variation within a species. There are a number of evolutionary mechanisms that may be responsible for polymorphism including genetic drift, gene flow and natural selection. Cepaea nemoralis is a common land snail found across Western Europe, which is popular in studies of polymorphism due to its range of easily observable phenotypic traits. These include differences in shell colour (brown, pink or yellow) as well as the presence, number (up to five bands) and appearance of band patterns. The age of C. nemoralis can be estimated by the degree of pigmentation at the lip of the shell, with darker lips being found in mature individuals. Snails are model organisms because they are slow moving so they are easy to sample and it can be assumed that migration will have little influence on any polymorphism observed. Additionally snails are valuable in polymorphic research because upon death, their shells remain in the area and can still be sampled. A supergene is responsible for polymorphic traits in C. nemoralis shells. This is where genes responsible for colour, presence or absence of bands, lip colour and type of band pigmentation are found located close together on a chromosome. Due to the close proximity of the genes that form the supergene, recombination is rare.
Referring back to previous experiments, one cannot help but notice the exploration of competition between sub-species. In one study, the gene frequencies of both C. nemoralis and C. hortensis (a similar species of snail) were observed based on geographical location. In the south of Britain, C. hortensis is almost completely excluded by C. nemoralis from dunes. Conversely C. hortensis is abundant in the north limit, compared to the sparseness of C. nemoralis. These observations show clear illustration of competition between the two and how both species cannot co-habit. Competitive exclusion has also been suggested on a smaller scale for other habitats. However, no further exploration has gone into carrying out the removal of one species in order to investigate the niche expansion by the other .Focusing more specifically on areas of vegetation, studies have been carried out to explore a possible stochastic element in competition. When observing population numbers it was evident that in some places, C. nemoralis is abundant, where C. hortensis is confined and restricted to woods. However in other areas, it was discovered that the opposite pattern was found. In terms of polymorphism, experimentation was carried out in Oxfordshire regarding the distribution of C. nemoralis, by shells colour and bands. In woodland, it was seen that pink shell populations had very high frequencies as compared to other colours. In other locations however, strategies are reversed. In the woodland, an increase in the frequency of these pink snails causes a response in the visual selection in the woodland. Moreover the increase is coupled with a general increase in the number of band fusion frequencies. Further research on genetic backgrounds have also shown a relationship between the local populations of C. nemoralis and interations between shell banding loci, which can also be subjected to natural selection.
We have also looked at similar experiments which focused specifically on C. nemoralis and their thermal environments. Many scientists looked at the effect the phenotype would have on their thermal environment, and deduced it could be significant enough to show a change in morph frequency ; for example: in warmer climates you are more likely to find a higher frequency of live C .nemoralis of yellow shells, due to the fact they are less resistant to overheating, while the pink and brown shells are more efficient at absorbing heat radiation so are more suited to colder climates. The idea of visual selection was also explored by many. The song thrush Turdus ericetorum is a known predator of Cepaea, therefore it is possible to look at the morph frequencies of C.nemoralis compared to its environment. It was found that in more covered slightly denser areas of woodland e.g. dead leaves and thick vegetation, higher frequencies of pink and brown shells were found, compared to open grassland where yellow shells were the majority. This reverts back to the basic need to survive; therefore camouflage could work in their favour.
The aim of the experiment is to find out how the morphology of the population changed in between covered and exposed areas. Covered areas are described as having unexposed soil, long dense vegetation and more saturated soil. It presumed that these areas would be colder because there is less direct sunlight hitting the soil. Exposed areas are described as having shorter vegetation, less saturated soil and dry ground which is exposed to more sunlight. This means that exposed areas receive more direct sunlight. In order to test this theory, two transects were laid out. Each transect contained both covered and exposed areas and snails were counted within the 3 by 3 metre quadrats to compare the differences in populations. Within each area we looked at snail colour and number of bands to see if there was a relationship between the morphology of the snail and its environment. We hypothesise that there will be multiple niche polymorphism where different phenotypes will be more suited for different niches i.e. darker snails will be found in covered areas.
DISCUSSION
Having used the Chi squared test to analyse the data, no significant difference was found at the 5 percent level between the colour of C.nemoralis shells found at uncovered sites compared to those found at covered sites. The results of this experiment differ somewhat to previous experiments conducted on a similar theme. Perhaps most notably, Steve Jones’ investigation into the cause of differentiation between snail shell colouration along a transect of elevation, which determined that shell colouration should become darker (brown colouration) as elevation decreased, whereas lighter colouration (yellow and pink) was discovered towards the peak of the elevation. This differs from the results of this experiment, which showed no significant difference between the distributions of snails along an elevation transect due to shell colour. This comparison suggests an issue with the experimental design. Samples were gathered from an area that is popular in the study of polymorphism in the C.nemoralis and a consequence of this is that individuals sampled from the area, may not be returned to the exact place from which they were found. This suggests that the lack of significant difference observed, may be due to disruption of the natural distribution of C.nemoralis (caused by human interference). In addition, the lack of significant difference found, may be also be an indication that the sample size used, was too small for significance to be detected so a wider sampling area through strategic planning would have diminished any anomalous results. Going over the selected sites at different times of the day as a form of repetition could have also improved the reliability of our results. Other h effects like climate, season and even type of vegetation could have been taken into consideration as they could have affected the niche of the snails. Looking at accuracy, a transect of selected volume could have improved the quality of our findings and may have even improved productivity on the day. This means that the alternative hypothesis should not be rejected completely, unless the study is repeated using a greater number of samples, gathered from an area that is not subject to disruption, and the lack of significant difference remains.
B. Dixon-Naraine, C. Kuffour , J. Jones-Alleyne, J. Rogers, K.Oseni, V. Julien
Queen Mary University of London
INTRODUCTION
Polymorphism is the phenomena where two or more phenotypes are observed in the same species. It is a natural process that promotes variation within a species. There are a number of evolutionary mechanisms that may be responsible for polymorphism including genetic drift, gene flow and natural selection. Cepaea nemoralis is a common land snail found across Western Europe, which is popular in studies of polymorphism due to its range of easily observable phenotypic traits. These include differences in shell colour (brown, pink or yellow) as well as the presence, number (up to five bands) and appearance of band patterns. The age of C. nemoralis can be estimated by the degree of pigmentation at the lip of the shell, with darker lips being found in mature individuals. Snails are model organisms because they are slow moving so they are easy to sample and it can be assumed that migration will have little influence on any polymorphism observed. Additionally snails are valuable in polymorphic research because upon death, their shells remain in the area and can still be sampled. A supergene is responsible for polymorphic traits in C. nemoralis shells. This is where genes responsible for colour, presence or absence of bands, lip colour and type of band pigmentation are found located close together on a chromosome. Due to the close proximity of the genes that form the supergene, recombination is rare.
Referring back to previous experiments, one cannot help but notice the exploration of competition between sub-species. In one study, the gene frequencies of both C. nemoralis and C. hortensis (a similar species of snail) were observed based on geographical location. In the south of Britain, C. hortensis is almost completely excluded by C. nemoralis from dunes. Conversely C. hortensis is abundant in the north limit, compared to the sparseness of C. nemoralis. These observations show clear illustration of competition between the two and how both species cannot co-habit. Competitive exclusion has also been suggested on a smaller scale for other habitats. However, no further exploration has gone into carrying out the removal of one species in order to investigate the niche expansion by the other .Focusing more specifically on areas of vegetation, studies have been carried out to explore a possible stochastic element in competition. When observing population numbers it was evident that in some places, C. nemoralis is abundant, where C. hortensis is confined and restricted to woods. However in other areas, it was discovered that the opposite pattern was found. In terms of polymorphism, experimentation was carried out in Oxfordshire regarding the distribution of C. nemoralis, by shells colour and bands. In woodland, it was seen that pink shell populations had very high frequencies as compared to other colours. In other locations however, strategies are reversed. In the woodland, an increase in the frequency of these pink snails causes a response in the visual selection in the woodland. Moreover the increase is coupled with a general increase in the number of band fusion frequencies. Further research on genetic backgrounds have also shown a relationship between the local populations of C. nemoralis and interations between shell banding loci, which can also be subjected to natural selection.
We have also looked at similar experiments which focused specifically on C. nemoralis and their thermal environments. Many scientists looked at the effect the phenotype would have on their thermal environment, and deduced it could be significant enough to show a change in morph frequency ; for example: in warmer climates you are more likely to find a higher frequency of live C .nemoralis of yellow shells, due to the fact they are less resistant to overheating, while the pink and brown shells are more efficient at absorbing heat radiation so are more suited to colder climates. The idea of visual selection was also explored by many. The song thrush Turdus ericetorum is a known predator of Cepaea, therefore it is possible to look at the morph frequencies of C.nemoralis compared to its environment. It was found that in more covered slightly denser areas of woodland e.g. dead leaves and thick vegetation, higher frequencies of pink and brown shells were found, compared to open grassland where yellow shells were the majority. This reverts back to the basic need to survive; therefore camouflage could work in their favour.
The aim of the experiment is to find out how the morphology of the population changed in between covered and exposed areas. Covered areas are described as having unexposed soil, long dense vegetation and more saturated soil. It presumed that these areas would be colder because there is less direct sunlight hitting the soil. Exposed areas are described as having shorter vegetation, less saturated soil and dry ground which is exposed to more sunlight. This means that exposed areas receive more direct sunlight. In order to test this theory, two transects were laid out. Each transect contained both covered and exposed areas and snails were counted within the 3 by 3 metre quadrats to compare the differences in populations. Within each area we looked at snail colour and number of bands to see if there was a relationship between the morphology of the snail and its environment. We hypothesise that there will be multiple niche polymorphism where different phenotypes will be more suited for different niches i.e. darker snails will be found in covered areas.
DISCUSSION
Having used the Chi squared test to analyse the data, no significant difference was found at the 5 percent level between the colour of C.nemoralis shells found at uncovered sites compared to those found at covered sites. The results of this experiment differ somewhat to previous experiments conducted on a similar theme. Perhaps most notably, Steve Jones’ investigation into the cause of differentiation between snail shell colouration along a transect of elevation, which determined that shell colouration should become darker (brown colouration) as elevation decreased, whereas lighter colouration (yellow and pink) was discovered towards the peak of the elevation. This differs from the results of this experiment, which showed no significant difference between the distributions of snails along an elevation transect due to shell colour. This comparison suggests an issue with the experimental design.
Samples were gathered from an area that is popular in the study of polymorphism in the C.nemoralis and a consequence of this is that individuals sampled from the area, may not be returned to the exact place from which they were found. This suggests that the lack of significant difference observed, may be due to disruption of the natural distribution of C.nemoralis (caused by human interference). In addition, the lack of significant difference found, may be also be an indication that the sample size used, was too small for significance to be detected so a wider sampling area through strategic planning would have diminished any anomalous results. Going over the selected sites at different times of the day as a form of repetition could have also improved the reliability of our results. Other h effects like climate, season and even type of vegetation could have been taken into consideration as they could have affected the niche of the snails. Looking at accuracy, a transect of selected volume could have improved the quality of our findings and may have even improved productivity on the day. This means that the alternative hypothesis should not be rejected completely, unless the study is repeated using a greater number of samples, gathered from an area that is not subject to disruption, and the lack of significant difference remains.